Finland and Sweden will join NATO leaders at the top table for the first time on Friday as Russia's military build-up pushes the two countries closer to the western military alliance.

At NATO's summit dinner in Warsaw, Russia's neighbors Sweden and Finland will sit with U.S. President Barack Obama, NATO Secretary-General Jens Stoltenberg and other leaders to a background of deepening cooperation that includes taking part in military exercises and missions.

"We are inviting Finland and Sweden because they are really close friends of NATO," Stoltenberg said, noting the two countries' location on the Baltic Sea, where Russia is increasing its military presence.

But Sweden, and even more so Finland, which has more than 1,300 kilometers (800 miles) of border with Russia, know any move to join the alliance would create a backlash in Moscow.

"We would have to go through a phase of Russia's initial reaction that wouldn't be positive," Finland's former prime minister, Alexander Stubb, who supports joining NATO, told Reuters.

Russian President Vladimir Putin visited Helsinki last week and hinted he would move troops closer to Finland's border if it joined.

Moscow says moves by Helsinki and Stockholm towards closer ties with NATO are of "special concern" to Russia, which has vehemently opposed NATO's eastward expansion.

Moscow has been building up military capabilities in its Baltic enclave of Kaliningrad. It has angrily criticized the stationing of NATO's anti-missile shield in eastern Europe.

Sweden and Finland have both complained of incidents involving Russian submarines and aircraft in the Baltic Sea region. They have responded by tightening bilateral military cooperation and fostering more ties with NATO.

"Smoking without inhaling"

They are likely to remain very close partners of NATO without becoming full members, something Carnegie Europe analyst Andrei Kolesnikov described as "smoking without inhaling".

They take part in NATO military drills, including parts of those scripted around Article V which lays down the principle of collective security, but are not bound by it as non-members.

Sweden was involved in NATO's 2011 air campaign in Libya and both Nordic neighbors sent troops to the alliance's mission in Afghanistan.

"They cannot be part of decision-making but they are definitely part of decision-shaping. The fact that the leaders will discuss Russia at the summit dinner, and that Sweden and Finland take part, sends a message," said a NATO official.

Finland and Sweden cannot be part of the alliance's contingency planning. Some in NATO want to see Helsinki and Stockholm firmly in.

"It is an issue in case of a conflict in the Baltic Sea region. We would need to work with Sweden and Finland, especially to use their airspace," said a NATO diplomat.

"We can talk now and imagine how that would work. But eventually it would be their political decision, we have no guarantees."

Some in Sweden and Finland view their current ties with NATO as already too strong, and accuse their governments of trying to get them into the alliance through the back door.

An SvD/SIFO opinion poll showed 49 percent of Swedes opposed joining NATO, with 33 in favor. Most Finns are against entering, and a government report said in April any such move would trigger a crisis with Russia.